The present invention relates to an AFC (Auto Frequency Control) circuit and, more particularly, to a frequency offset correction system and correction method used in the AFC circuit of a CDMA (Code Division Multiple Access) receiver.
As shown in FIG. 5, a conventional CDMA receiver comprises a down converter 11 for receiving an RF (Radio Frequency) signal and converting it into an IF (Intermediate Frequency) signal, a despread unit 12 for spectrum-despreading the IF signal output from the down converter 11, and a demodulator 13 for demodulating an output signal from the despread unit 12 to generate received data.
A CDMA received signal is a signal prepared by modulating a signal into BPSK (Binary Phase Shift Keying) signals by data and pilot signal data of I-Channel (In-phase component) and Q-Channel (Quadrature component), superimposing these signals to modulate them into QPSK (Quadrature Phase Shift Keying) signals, and modulating the resultant signals by a spread code.
FIGS. 6A to 6D show the frame structure of CDMA received data. As shown in FIG. 6A, one frame of CDMA received data is constituted by Frame#1, Frame#2, Frame #3, . . . , Frame#72, and has one period TSuper=720 ms. As shown in FIG. 6B, Frame#3 is comprised of Slot#1, Slot #2, Slot#3, . . . , Slot#15, and has one period Tf=10 ms.
As shown in FIGS. 6C and 6D, Slot#3 is made up of I-channel and Q-Channel, and has one period Ts=0.666 ms. I-Channel has Ndata-bit data, and Q-Channel has Npilot-bit pilot signal data.
Table 1 shows the relationship between the bit rates and the number of bits of I-Channel and Q-Channel in FIGS. 6C and 6D.
TABLE 1I-ChannelQ-ChannelChannelChannelBit RateNdataBit RateNpilotn/m =(kbps)(bits)(kbps)(bits)Ndata/Npilot1510151013020151026040151041208015108240160151016480320151032960640151064
As shown in Table 1, the channel bit rate of Q-Channel is fixed to, e.g., 15 kbps, and the number (Npilot) of bits of pilot signal data is also fixed to, e.g., 10. To the contrary, the channel bit rate of I-Channel is variable, and takes, e.g., 15, 30, 60, 120, 240, 480, and 960 kbps. In correspondence with them, the number (Ndata) of bits of data is also variable, and takes, e.g., 10, 20, 40, 80, 160, 320, and 640.
The AFC circuit of a COMA demodulator comprises a frequency offset correction apparatus for correcting the frequency offset of a received signal. The frequency offset correction apparatus stores pilot signal data (fixed bit rate) of Q-Channel and data (variable bit rate) of I-Channel for one slot. The AFC circuit estimates the frequency offset value from the stored pilot signal data of Q-Channel, and corrects the frequency offset of the received signal at the bit rate of the pilot signal data.
Frequency offset correction of removing a frequency offset from a received signal and performing modulation and the like is used not only in the CDMA but also in another system, as disclosed in Japanese Patent Laid-Open No. 6-30070. To increase the precision of frequency offset correction for a received signal, there is proposed a method of correcting the frequency offset at the bit rate of data of I-Channel using the frequency offset value of estimated pilot signal data.
In the conventional frequency offset correction method, when the channel bit rate of data of I-Channel is higher than that of pilot signal data of Q-Channel, e.g., the truncation error of the frequency offset in obtaining a phase shift Δθ per bit increases.
This will be described in detail. To perform integrating operation for a frequency offset estimated from pilot signal data, phase shifts Δθ are added within one slot. As the channel bit rate of data of I-Channel is higher, the number of channel bits is larger. Thus, the number of phase shifts Δθ added within one slot increases, and the influence of the error of the frequency offset caused by integrating operation increases.
In the conventional frequency offset correction method, when the frequency offset of pilot signal data decreases, the change amount of the phase shift per symbol decreases. Particularly when the angular frequency of pilot signal data of Q-Channel is processed at a fixed point, the truncation error of the frequency offset further increases.